The invention relates generally to projection displays, and more particularly, relates to a compact prismatic light beam homogenizer for projection displays of the electronic valve type.
High intensity light sources that produce uniform and collimated light are increasingly important for projection display systems. One of the most common and efficient methods of collecting light from a high intensity arc source, such as a metal-halide lamp, employs a parabolic or elliptic reflector, with axial mounting of the lamp. For reflective collectors such as this and others, brightness non-uniformities at the light valve and projected image occur because of three primary factors. One factor is reduction of projected brightness in peripheral regions of the reflector. Another factor is the shadowing effect of the lamp electrode, envelope, and electrical lead near a central region of the reflector. A third factor is a vertex hole in the reflector, through which the lamp is conventionally mounted.
Several techniques have been employed to increase light uniformity from light sources and reflecting collectors, particularly for projection display applications. One technique uses a separated pair of multi-lens arrays, positioned between a light source and a light valve. This technique was initially described in U.S. Pat. No. 2,183,249, of Schering, and U.S. Pat. Nos. 2,186,123 and 2,326,970, of Rantsch. More recently, variations of the technique have been described, for example, in European Patent Application 512893, of Lehureau, U.S. Pat. No. 4,769,750, of Matsumoto, European Patent Application 646828, of Nakayama, and U.S. Pat. No. 5,418,583, of Masumoto.
Another technique uses a non-imaging light pipe or light guide to form a homogenized light beam at an exit aperture. This technique was described by Marshall in "Tapered light guide condenser: a design approach", SPIE Proceedings, Vol. 176, 1979, pp. 161-167. The non-imaging reflectors of the technique are further described in U.S. Pat. No. 5,146,258, of Duwaer, Japanese Patent 380221, of Casio, U.S. Pat. No. 5,459,592, of Shibatani, Japanese Patent 7301800, of Fujitsu, and European Patent Application 676902, of Dolgoff.
Another related approach uses multiple scattering within a cavity completely enclosing a light source. The light emerges from an exit aperture. The approach was described in WO Publication 91/18315.
Yet another approach to providing a more uniform light beam from a reflecting parabolic collector includes placing an aspheric correcting lens with a strong central gradient between a light source and a light valve. The approach was described in European Patent Application 545052, of Mitsutake. Other types of compensating lenses or beam deflecting prisms have been described in U.S. Pat. No. 5,459,592, of Shibatani, Japanese Patent 7321003, of Ricoh, and Japanese Patent 8201759, of NEC.
Notwithstanding the foregoing techniques, various disadvantages are presented by the conventional technology. For instance, refractive beam homogenizing devices can introduce additional chromatic dispersion into the projection optics. Reflective light pipe beam homogenizers or light integrators, as well, are not very efficient in transmission, and are usually not very compact. Likewise, beam deflecting prisms, positioned between a light source and light valve, can radically change the incident angles of light at the light valve, thus, causing problems in focusing the light into the projection lens.
Therefore, what is needed is a prismatic light beam homogenizer for application in projection displays, for example, with a light valve or LCD light source, which homogenizer has a design that overcomes many of these disadvantages.